1. Field of the Invention
This invention relates to an extensible clasp for use with a flexible wristlet, e.g. a bracelet, as for a watch.
Wristlets, particularly those of watches, should be easily closable and not be liable to slip away when forming the loop round a wrist.
For this reason, a wristlet should preferably be continuous as it cannot then slip off the wrist. The length of the wristlet must then be variable to enable it first to be slipped over the hand and then shortened to fit snugly round the wrist.
2. Prior Art
One known bracelet construction consists of contiguous metal segments that contain small springs enabling the bracelet to lengthen when pulled. For this the springs must be very small with the result that they are over exerted and short lived.
To remedy this, hinged clasps have been proposed for use with a metal bracelet comprising a pair of flexible but inextensible portions. In this improved continuous arrangement, the clasp comprises rigid, hingedly connected, portions that can be folded over each other. A great many such clasps have been designed. One such clasp comprises three rigid portions with the first folding under the second whilst the third, acting as a cover, engages over the second and locks on the first.
One notable improvement was achieved with the clasp disclosed in Swiss Patent Specification No. 568035, which combines improved aesthetics with improved ease of closure and safety.
This clasp is made up of two non deformable portions hinged to each other, in which the second portion is wholly covered by a part of the flexible bracelet whilst both segments overlie each other in the closed position. When closed, the bracelet appears to have a continuous line except for a plate which hides a lateral locking device.
The bracelet is elegant, the two folded portions of the clasp are invisible and the closing action is and looks reliable. However, between the open and closed positions of the bracelet, the difference in bracelet length is limited, particularly so when the clasp is made up of short portions as is usual for small, women's wrists.
Other designers have proposed bracelets with clasps that again comprise two rigid portions but in which, at the end of one portion, an elastic hinge arrangement, involving spring means, is provided within the thickness of the bracelet. Such a construction is detrimental to both the trustworthiness and the aesthetics of the bracelet.
The specification of Italian Patent Application No. 21184A/81 discloses an extensible bracelet fitted with a clasp having two portions of which one is made up of two parts that slide one within the other. Small springs are provided to return the two parts into a contracted position and means are provided for securing them together in that position. The clasp is thus extensible and reliable but still suffers from two drawbacks: the small springs are soon subject to fatigue and the two sliding parts continuously tend to move to their contracted position, already when slipping the bracelet over a hand. Further, the scope for adjustment is small.